The invention relates to a method of manufacturing pressed panel-shaped products comprising chips of lignocellulosic annual plants and a binder.
In the production of multi-layered plates or panels of lignocellulose-containing materials, it is known to use isocyanate as a binder. The use of isocyanate as a binder for panel products as well as the advantages and disadvantages resulting therefrom are described in the special publication xe2x80x9cIsocyanate as a binder for particle boardxe2x80x9d by Dr. G. Loew, an Engineer H. I. Sachs of Bayer A G, from 1977, on page 479. From this it is evident that polyisocyanates adhere onto metals under pressure and heat. As a solution possibility to these problems, the use of liquid separating agents as well as the use of papers, veneers, as well as dust or conventionally adhesive bonded or glued chip cover layers is suggested.
A method for the production of multi-layered panel materials of a mass of wood chips, wood fibers, or of lignified raw materials, impregnated with isocyanate, is known from the Austrian Patent 270,189. Furthermore it is known from this publication, to use a mixture of isocyanate as well as urea-, melamine-, and phenol formaldehyde resin glue or adhesive as a binder, or for example, to bind the chips in the cover layer with melamine resin and the chips of the middle layer with pure isocyanate solution, or vice versa. By the use of a mixture of the above named binders, a strongly increased moisture resistance and a considerable reduction of the swelling of the panel products upon the penetration of moisture are achieved.
From the U.S. Pat. No. 5,779,955, a method for the production of panel materials is known, in which particles of plant-related products are mixed with isocyanate as a binder as well as water, as the panel material. In order to avoid the problems of the adhering of the mats during the pressing, which problems arise with the use of isocyanate as a binder, the underlayer is covered with a liquid separating agent before the spreading of the mixture. After the spreading process, a separating agent is similarly applied onto the top surface of the laid-down mixture. Thereafter, the hot pressing of the formed body is carried out.
Moreover, a method for the production of multi-layered chip panels by hot pressing of woodchips impregnated with binder is known from the German Patent Laying-Open Document 38 20 376. In this context, isocyanate is used as a binder in combination with formaldehyde binding agents, such as urea or the like, is used as a binder in the core layer. In the cover layers, isocyanate is used as a binder In order to avoid the already described problem of the adhesion of the mats during the pressing, separating cover layers are provided on the cover layers. These separating cover layers are bonded with phenolic resin and are again removed after the hardening or curing.
Object of the present invention is to provide a method for the production of multi-layered panel-shaped products of lignocellulose-containing annual plants, in which especially the problems of the adhesion in the use of isocyanate as a binder are avoided.
This object is achieved according to the invention in a method wherein a cover layer material of chips of at least one lignocellulose-containing annual plant is separated into a cover layer material fine fraction and a cover layer material coarse fraction, the cover layer material fine fraction is premixed with an adhesion promoter and/or a wetting agent and/or acids and/or bases, and next is glued with amino and/or phenolic plastics, the cover layer material coarse fraction is glued with isocyanate or mixtures of isocyanate and amino and/or phenolic plastics, and next the cover layer material fine fraction, the cover layer material coarse fraction, and the middle layer material are spread in respective layers to form a five-layered formed body, whereby outermost cover layers of the formed body consist of the cover layer material fine fraction that is glued with amino and/or phenolic plastics.
By means of the separation or division of the cover layer material into a fine fraction and a coarse fraction described in the method according to the invention, it is possible to glue the fine fraction with amino and/or phenolic plastics through a pretreatment of the fine fraction with supplemental or additive agents. Due to the higher degree of comminution of the fine fraction, the wetting capacity of the particles of lignocellulose-containing annual plants with amino and/or phenolic plastics as a binder is advantageously influenced. The addition of wetting agent additionally improves the wetting characteristics. A further addition of an adhesion promoter during the premixing achieves a clear improvement of the adhesion or bonding characteristics of the particles. Since the outer cover layers are glued with amino and/or phenolic plastics, the problems of the adhesion of isocyanate glued chips on the panel or plate surfaces during the pressing are avoided. In the method according to the invention, no additional separating agents or separating layers are necessary, so that additional costs are avoided.
The cold adhesiveness present in the outer cover layers achieves a surface stability of the outer layers of the spread formed body, so that the multi-layered formed body may be transported over the individual transfer area in the forming or extrusion line to the press inlet without damaging the previously achieved cover layer quality.
A use of a binder mixture of isocyanate and phenolic and/or amino plastics leads to a cost reduction, in comparison to the exclusive use of isocyanate as a binder for the middle layer material as well as the cover layer material coarse fraction, due to a reducing proportion of isocyanate.